Plastic materials including plastic sheeting, synthetic fibers and the like are generally considered to be hazardous materials from the standpoint of flammability; and in many cases it is the practice to add various flame retardants to the plastic materials to reduce flammability during combustion. Recently, however, the propensity of plastics to create large volumes of smoke, when combusted, has come to be regarded as at least as hazardous, if not more so, than its flammability. Increasing concern with consumer safety and several tragic accidents have contributed to the recognition of smoke evolution as a significant hazard. In many instances of fire, the resulting fatalities have been due to smoke which has prevented safe evacuation of a building due to lack of visibility; or to the toxic gases generated during combustion of the plastic.
As mentioned above, extensive studies have been made in the area of flame-retardant additives for halogen-containing plastic compositions, examples of which are antimony trioxide, antimony pentaoxide, antimony-silica oxide and other organic and inorganic compositions of antimony. Zinc borate has been used also as a flame retardant but so far as is known never as a smoke suppressant and always at relatively high levels as for example, from 3 to 8 parts per hundred parts resin since lesser amounts are ineffective as a flame retardant. Moreover, the concept of suppressing smoke as well as retarding flammability is disclosed in U.S. Pat. No. 3,723,139, Larkin et al who disclose the addition of antimony oxide in combination with sodium antimonate to polyvinyl chloride plastics both to minimize flammability and to suppress smoking.
Copending application Ser. No. 426,492, filed Dec. 20, 1973, discloses and claims halogen-containing plastic compositions which contain various zinc salts in amounts from 0.2 to 1 part calculated as zinc oxide per hundred parts of polymer resin, which are also useful as smoke suppressants.
However, when these zinc salts are used at higher concentration than that specified above, for retarding fire in addition to smoke, the zinc salts tend to degrade the plastic composition when they are originally prepared by conventional high temperature processes.